E & M Signaling

A simple bi-directional signaling method developed as a standard interface connecting a phone switching system (such as a PBX) to an interoffice trunk circuit. Normally, there is a single phone number associated with a standard analog phone line, such as the one in your home. With Direct Inward Dial (DID) and Ear (or Earth) and Mouth (or Magnet) or E&M circuits, more than one number may be associated with one line. DID trunks usually connect customers with phone companies while E&M circuits can be used to provide internal connections between PBXs. Both DID and E&M type trunks provide similar functions but with different technology. With both DID and E&M circuits, the dialed phone number is prefixed to the call so the equipment can determine the number called.

With E&M signaling, the signaling between the trunk circuit and the associated signaling unit is done over two leads providing full-time, two-way, two-level supervision. These signaling leads are kept separate from the speech path. Originally, in the days of telegraphy, the E ( Ear ) and M ( Mouth ) paths occupied a single wire, with a relay contact closure or key on the sending side drawing current from a detector on the receiving side consisting of an electromagnet attached to a battery. These days, two wires are used per path so that the sending side draws current from itself, thus reducing induced noise in neighboring circuits.

Although E&M signaling is popular in North America, in foreign countries one comes across local technologies...